1. Field of the Invention
The invention relates to methods for coating stents. In particular, the methods are directed to coating a stent mounted on a balloon of a catheter assembly.
2. Description of the Background
Stents are being modified to provide drug delivery capabilities. A polymeric carrier, impregnated with a drug or therapeutic substance is coated on a stent. The conventional method of coating is by, for example, applying a composition including a solvent, a polymer dissolved in the solvent, and a therapeutic substance dispersed in the blend to the stent by immersing the stent in the composition or by spraying the composition onto the stent. The solvent is allowed to evaporate, leaving on the stent strut surfaces a coating of the polymer and the therapeutic substance impregnated in the polymer. The dipping or spraying of the composition onto the stent can result in a complete coverage of all stent surfaces, i.e., both luminal (inner) and abluminal (outer) surfaces, with a coating. However, having a coating on the luminal surface of the stent can have a detrimental impact on the stent's deliverability as well as the coating's mechanical integrity. Moreover, from a therapeutic standpoint, the therapeutic agents on an inner surface of the stent get washed away by the blood flow and typically can provide for an insignificant therapeutic effect. In contrast, the agents on the outer surfaces of the stent are in contact with the lumen, and provide for the delivery of the agent directly to the tissues. Polymers of a stent coating also elicit a response from the body. Reducing the amount to foreign material can only be beneficial.
Briefly, an inflatable balloon of a catheter assembly is inserted into a hollow bore of a coated stent. The stent is securely mounted on the balloon by a crimping process. The balloon is inflated to implant the stent, deflated, and then withdrawn out from the bore of the stent. A polymeric coating on the inner surface of the stent can increase the coefficient of friction between the stent and the balloon of a catheter assembly on which the stent is crimped for delivery. Additionally, some polymers have a “sticky” or “tacky” consistency. If the polymeric material either increases the coefficient of friction or adherers to the catheter balloon, the effective release of the stent from the balloon after deflation can be compromised. If the stent coating adheres to the balloon, the coating, or parts thereof, can be pulled off the stent during the process of deflation and withdrawal of the balloon following the placement of the stent. Adhesive, polymeric stent coatings can also experience extensive balloon sheer damage post-deployment, which could result in a thrombogenic stent surface and possible embolic debris. The stent coating can stretch when the balloon is expanded and may delaminate as a result of such shear stress.
Accordingly, it is advantageous to eliminate the coating on the inner surface of the stent. Post crimping coating processes have been proposed for elimination of the coating on the inner surface of the stent. Briefly, subsequent to the mounting of the stent on the balloon, the stent can be dipped in the coating composition or the composition can be sprayed on the stent. Even though application of coating on the inner surface of the stent is eliminated, the coating is also deposited on the surface of the balloon between the stent struts. With this type of coating, the problems of adhesion of the stent to the balloon and formation of coating defects upon expansion, deflation and withdrawal of the balloon are not eliminated, and in effect, such problems could be further exasperated.
Coating of the stent prior to mounting of the stent on the balloon can also damage the coating on the outer surface of the stent. Stent crimping tools can cause coating defects on the stent by applying too much pressure at various directions to a soft polymeric coating. Harder or brittle polymers can have coating failure or crack under crimping pressure. Stent crimping is a critical step in manufacturing in that stent retention depends on it. Stent crimping is the act of affixing the stent to the delivery catheter or delivery balloon so that it remains affixed to the catheter or balloon until the physician desires to deliver the stent at the treatment site. Current stent crimping technology is sophisticated. A short time ago, one process used a roll crimper. This damaged many polymer coatings due to its inherent shearing action. Next came the collet crimper using metal jaws that are mounted into what is essentially a drill chuck. The jaws move in a purely radial direction. This movement was not expected to shear the coating, because it applied forces only normal to the stent surface. But some stent geometries require that stent struts scissor together during crimping. In those geometries, even if the crimper imposes only normal forces, the scissor action of the stent struts imparts shear. Finally, the iris or sliding-wedge crimper imparts mostly normal forces with some amount of tangential shear.
To use a roll crimper, first the stent is slid loosely onto the balloon portion of the catheter. This assembly is placed between the plates of the roll crimper. With an automated roll crimper, the plates come together and apply a specified amount of force. They then move back and forth a set distance in a direction that is perpendicular to the catheter. The catheter rolls back and forth under this motion, and the diameter of the stent is reduced. The process can be broken down into more than one step, each with its own level of force, translational distance, and number of cycles. With regard to a stent with a drug delivery coating, this process imparts a great deal of shear to the stent in a direction perpendicular to the catheter or catheter wall. Furthermore, as the stent is crimped, there is additional relative motion between the stent surface and the crimping plates. As a result, this crimping process tends to damage the stent coating.
The collet crimper is equally conceptually simple. A standard drill-chuck collet is equipped with several pie-piece-shaped jaws. These jaws move in a radial direction as an outer ring is turned. To use this crimper, a stent is loosely placed onto the balloon portion of a catheter and inserted in the center space between the jaws. Turning the outer ring causes the jaws to move inward. An issue with this device is determining or designing the crimping endpoint. One scheme is to engineer the jaws so that when they completely close, they touch and a center hole of a known diameter remains. Using this approach, turning the collet onto the collet stops crimps the stent to the known outer diameter. While this seems ideal, it can lead to problems. Stent struts have a tolerance on their thickness. Additionally, the process of folding non-compliant balloons is not exactly reproducible. Consequently, the collet crimper exerts a different amount of force on each stent in order to achieve the same final dimension. Unless this force, and the final crimped diameter, is carefully chosen, the variability of the stent and balloon dimensions can yield stent coating or balloon damage.
Furthermore, although the collet jaws move in a radial direction, they move closer together as they crimp. This action, combined with the scissoring motion of the struts, imparts tangential shear on the coatings that can also lead to damage. Lastly, the actual contact surfaces of the collet crimper are the jaw tips. These surfaces are quite small, and only form a cylindrical surface at the final point of crimping. Before that point, the load being applied to the stent surface is discontinuous.
In the sliding wedge or iris crimper, adjacent pie-piece-shaped sections move inward and twist, much like the leaves in a camera aperture. This crimper can be engineered to have two different types of endpoints. It can stop at a final diameter, or it can apply a fixed force and allow the final diameter to float. From the discussion on the collet crimper, there are advantages in applying a fixed level of force as variabilities in strut and balloon dimension will not change the crimping force. The sliding wedges impart primarily normal forces, which are the least damaging to stent coatings. As the wedges slide over each other, they impart some tangential force. But the shear damage is frequently equal to or less than that of the collet crimper. Lastly, the sliding wedge crimper presents a nearly cylindrical inner surface to the stent, even as it crimps. This means the crimping loads are distributed over the entire outer surface of the stent.
All current stent crimping methods were developed for all-metal stents. Stent metals, such as stainless steel, are durable and can take abuse. When crimping was too severe, it usually damaged the underlying balloon, not the stent. But polymeric coatings present different challenges.
The methods of the present invention provide for a solution by coating the outer surfaces of the stent post crimping or mounting of the stent to the balloon.